I. Field of the Invention
This invention relates to a power supply for driving an electronic load.
II. Description of the Prior Art
Off-line switching mode power supplies are commonly used in computers to achieve small size and high efficiency. The 115/230 Volts (AC) input power is rectified and passed through a regulating DC-DC converter to produce the various voltages needed for logic networks, disk drives, etc. A type of regulating converter in common use is the flyback or ringing choke type that stores energy in a multi-winding choke (power transformer) during the first half cycle and delivers this energy to the output load during the second half cycle. The storage and discharge portion of the duty cycle are generally controlled by a power transistor which acts as a switch. A common form of this flyback topology is the discontinuous flyback topology, in which the choke magnetic field is allowed to collapse completely after the second half cycle before a new cycle starts, leaving no energy stored in the choke. Two forms of discontinuous flyback topology are constant frequency, in which the cycle is started at regular fixed intervals, and free running, in which the new cycle starts immediately after the previous half cycle ends.
The power transistor used to switch the stored energy in the choke has a large peak power dissipated in the during the time required for transistor turnoff. In this short time, the peak power is dissipated near the surface of the transistor silicon chip and, if high voltage is present, on a small part of the surface. This dissipation of energy results in high peak temperatures and can produce a runaway condition in the silicon called second breakdown which usually destroys the transistor. This second breakdown limitation has resulted in special designs for high voltage switching power transistors and in snubber circuits to limit the rate of rise of voltage across the switching transistor during turnoff. High voltage across a transistor concentrates the current flow and causes a higher spot temperature. The snubber circuit allows the transistor to reach lower current before high voltage appears across the transistor.